Taste is important for detection of nutritional components or harmful components in foodstuffs. Taste perception of mammals is attained with the taste receptor cells contained in the taste buds present in the oral cavity. The received signals are transmitted from the taste receptor cells to taste nerves entering into the taste buds, and to the central nerve system. Tastes sensed by mammals are generally divided into five kinds of fundamental quality of tastes, that is, sweet taste, bitter taste, acid taste, salty taste, and umami taste, and these are called five basic tastes.
The receptors for these five basic tastes are being elucidated in accordance with the progress of researches in recent years (Non-patent documents 1 and 2). Identification and isolation of novel taste receptors enable novel methods of modulating taste perception. For example, search of highly affinitive agonists or antagonists using taste receptors enables screening for taste modulating substances. Such taste modulating substances may provide improvement or refinement of quality of tastes in various consumer products.
Salty taste, one of the five basic tastes, is involved in detection of sodium ions or other inorganic cations, and is very important for maintenance of internal homeostasis. It is considered that, as salty taste perception pathways, there are the amiloride-sensitive pathway which is inhibited by diuretic amiloride, and the amiloride-insensitive pathway which is not affected by amiloride (Non-patent document 3). Molecules involved in both the salty taste perception pathways exist and function in the taste receptor cells in the taste buds (Non-patent document 2).
It is considered that the amiloride-sensitive pathway is mediated by the epithelial sodium channel (ENaC). ENaC is composed of four kinds of subunits, α, β, γ, and δ, and functions as a hetero-multimer consisting of a combination of α, β and γ, or δ, β and γ (Non-patent document 13). However, although marked inhibition of the salty taste perception by amiloride is observed in rodents, such inhibition is observed in only a part of humans, and presence of a different receptor in the human salty taste perception mechanism is suggested. As described above, a significant part of the salty taste perception mechanism including receptors still remains unknown.
Excessive intake of salt from foodstuffs is considered as one of the risk factors of hypertension or cardiovascular system diseases, and there is movement of restricting salt consumption worldwide, including Japan (World Health Organization, Non-patent document 4).
There have been conventional techniques for decreasing intake of salt, for example, low-salt seasonings and low-salt food using potassium chloride as a substitute for sodium chloride. However, potassium chloride has a problem that it has a bitter taste and an irritating taste. Therefore, taste of food using potassium chloride is markedly inferior. In order to improve this drawback, there have been developed a seasoning composition in addition to potassium chloride, such as a mixture of ammonium chloride, calcium lactate, sodium L-aspartate, an L-glutamic acid salt and/or a nucleic acid type taste substance at a specific ratio (Patent document 1), a low sodium salty taste seasoning containing ascorbic acid (Patent document 2), a bitterness suppressing method using carrageenan (Patent document 3), and so forth. However, salt reduction techniques do not reach such a level that unpleasant tastes other than salty taste are eliminated, and salty taste intensity equivalent to that of sodium chloride is provided at the same time.
Furthermore, there are salt reduction methods using a salty taste enhancing substance, which does not reduce salty taste intensity even if sodium chloride is reduced. For example, it is known that a basic amino acid, especially L-arginine, has an effect of enhancing salty taste (Non-patent documents 5 and 6). As techniques applying the above knowledge, there have been developed a combination of L-arginine, L-aspartic acid and sodium chloride (Patent document 4), a taste improving agent using neutralized salts of a basic amino acid and citric acid (Patent document 5), and so forth. However, any technique that can sufficiently compensate insufficiency by reduction of sodium chloride has not been developed yet, in view of salt reduction effect, flavor, salty taste intensity and so forth.
Meanwhile, the Kv3 family is known as a family of potassium channel that is originally considered to function in a nerve cell that shows stimulation at high frequency, and release potassium ions when cell membrane potential is depolarized to −20 mV or higher to show outward current (Non-patent document 7). However, for these, only the function as a voltage-dependent potassium channel is known (Non-patent documents 8 to 11), and a function as a cation channel depending on extracellular sodium concentration around the resting membrane potential (about −60 to −80 mV) is not known at all.
In connection with taste, expression of Kv3.1 and Kv3.2 genes confirmed by PCR in taste cells isolated from rat fungiform papillae has been reported (Non-patent document 12). However, functions thereof in the taste cells are not known at all.